"There is a huge amount of scientific evidence showing that fruits and vegetables lower the risk of cancer and heart disease, but scientists have mostly been isolating single compounds like beta-carotene and vitamin C," Liu says. "Over the years, no single compound has been proven to have a protective effect by itself. An apple could have hundreds of phytochemicals. We think the combination is the important thing."
More than 900 different plant chemicals have been identified as components of different fruits, nuts, grains, and vegetables. Apples are rich in isoflavones and phenolics, but other widely studied phytochemicals include lycopene, found in tomatoes; carotenoids, found in carrots and citrus fruits; and allyl sulfides, found in garlic and onions. It is believed that various phytochemicals help prevent cell damage, prevent cancer cell replication, and decrease cholesterol levels.
Charles Halsted, MD, says evidence is mounting that suggests taking vitamin supplements, even in large doses, does not provide the health benefits of a healthy diet. Halsted edits the American Journal of Clinical Nutrition (AJCN) and is a professor of internal medicine at the University of California, Davis. He was not involved with the study, but reviewed it for WebMD.